Method for operating a four-stroke reciprocating internal combustion engine

ABSTRACT

In a four-stroke reciprocating-piston internal combustion engine, a method is carried out for operating the latter with a homogeneous lean basic mixture of air, fuel and retained exhaust gas and with compression ignition and direct fuel injection into a combustion space. In order to avoid ignition problems during compression ignition and in the case of a low load, in an activation phase, the retained exhaust gas is compressed in the region of the gas-exchange dead center and is subsequently expanded, and, in this phase of the work cycle, activation fuel is injected into the combustion space in order to stabilize main combustion.

BACKGROUND AND SUMMARY OF THE INVENTION

The invention relates to a method for operating a four-strokereciprocating-piston internal combustion engine of the generic type.

Reciprocating-piston internal combustion engines afford the possibility,during the compression ignition of homogeneous lean mixtures, of theformation of only small amounts of nitrogen oxides, along with highefficiency, when throttle control and a rich mixture are avoided.Compression ignition functions only with exhaust-gas retention in thecase of compression ratios which are customary in engines. At a lowengine load and low engine speed, and also when the engine is cold,however, even the rise in temperature due to exhaust-gas retention isnot sufficient for the reliable ignition of the fresh charge.

DE-A 195 19 663 describes a method for operating an internal combustionengine with compression ignition. Here, in a first step, a homogeneousand lean air/fuel mixture generated as a result of external mixtureformation is compressed to near the ignition limit. In a second step, anadditional quantity of the same fuel is finely atomized and, with wallcontact being avoided, is injected into the combustion space. The fuelinjected late forms a mixture cloud which ignites, since, because of thehigher fuel content, its ignition limit is below the compressiontemperature reached in the first step. These conditions apply to ahigher engine load and higher engine speed and to an engine which isrunning hot.

The object on which the invention is based is to provide a method of thegeneric type which allows reliable ignition and low-consumption andlow-pollutant combustion even at a low load and a low engine speed andwhen the engine is running cold.

By virtue of the method according to the invention, thereciprocating-piston internal combustion engine ignites the mixture ofair and fuel by virtue of the rise in temperature of the mixture. Therise in temperature of the fresh mixture is brought about by mixing withretained exhaust gas from the previous cycle and due to the subsequentgeometric compression of the closed-off maximum initial volume to aremaining residual volume. In the compressed volume, a temperature isestablished which brings the mixture to ignition. The combustion processwhich follows the compression ignition of the homogeneous lean mixtureis a process which is self-maintained maintained due to the energyreleased.

The exhaust gas occurs in the combustion space as a result of thecombustion of the fresh mixture. The energy released during combustionis dissipated as a result of expansion up to the maximumcombustion-space volume. Subsequently, an outlet member is opened andexhaust gas is expelled as a result of the reduction in thecombustion-space volume. While the expulsion operation is taking place,during the reduction in the combustion-space volume, the outlet membercloses and retains the exhaust gas. The latter is compressed to theminimum combustion-space volume. The retained exhaust gas has occurredduring combustion with air excess. The combustion phase is in the regionof maximum geometric compression between the compression phase and theexpansion phase. The number of thermodynamic phases of the four-strokeengine therefore amounts to five phases.

In order to broaden the operating range of an engine with thecompression ignition of homogeneous lean mixtures, a sixth thermodynamicphase, the activation phase, is interposed. During the compression ofthe retained exhaust gas, an activation fuel quantity is injected intothe air/exhaust-gas mixture and is distributed as homogeneously aspossible, together with the remaining air fractions, in the combustionspace. Thermal energy is supplied to the fuel by conduction andcompression, so that a chemical reaction and/or ignition is initiated.As a result of complete combustion of the activation fuel, the thermalenergy of the exhaust gas which has remained is increased in order toensure ignition in the next cycle. If combustion is incomplete, at leastthe chemical activity of the retained exhaust gas quantity is increased(the formation of radicals), without the temperature being raisedappreciably at the same time. In both cases, and in a situation wherethere is a mixture of these, it is possible to speak of activation. Agreater fresh-charge mass can be ignited by means of a smaller mass ofthe retained exhaust gas due to the activation of the latter.

In the case of combustion during exhaust-gas compression, there is arise in the state of pressure and temperature of the retained exhaustgas in the combustion space. This type of activation is also referred tocascadic combustion, since combustion takes place over two cycles. Thishas to be taken into account in a later inlet trigger time, in order toensure a negative pressure difference of the inlet valve due to a longerexpansion of the activated exhaust gas. There is no provision for anaccumulation of exhaust gas or for pushing the charge back into thesuction pipe, since the exhaust-gas quantity necessary for initiating areaction loses its specific enthalpy due to flow movements and mixingmovements.

At low engine speeds, that is to say with high heat losses at the wall,the aim is to achieve maximum activation of the exhaust gas. There isthe risk, in this case, that, during exhaust-gas compression, completecombustion will raise the pressure level in the combustion space in sucha way that the pressure difference between the surroundings and thecombustion space is subsequently not sufficient to suck in a sufficientquantity of fresh gas in the time which has remained when the inletvalves are open. In order to control the pressure and temperature of theretained exhaust gas in the combustion space, the injection point of theactivation fuel is varied. If injection is early, that is to say takesplace even during the reduction in the combustion-space volume, thetemperature of the mixture is increased due to compression, with theresult that a reaction is initiated, along with subsequent combustion.If injection of the activation fuel takes place in the region of theminimum combustion-space volume, the increase in this volume delayschemical activity (the formation of radicals), without a pronouncedtemperature rise being capable of being established in the exhaust gasas a result of self-maintaining reactivity.

The evaporation energy of the fuel injected late extracts thermal energyfrom the compressed retained exhaust gas. The evaporation energy mayalso be used to prevent uncontrolled after-ignition of the incompletelyburnt mixture mass during the compression of the retained exhaust gas.The chemical activation of the exhaust gas brings about an earlyinitiation of the main reaction in the subsequent complete compressionphase and combustion phase. The ignition point of the fresh chargeduring main combustion can be controlled by means of the point andquantity of activation injection.

In this way, an incomplete reaction during main combustion can beprevented by means of the control action exerted by a cooling ignitioninjection of activation fuel. The effects of integral influences due tothe transmission of wall heat on combustion can be seen over severalcycles. The influence of the transmission of wall heat within a cyclecan be compensated for a number of subsequent cycles by means of acontrolled action involving the variation in activation. This controlfor combustion stabilization requires its own control loop.

Incomplete combustion does not increase the emission of exhaust gas fromthe entire engine system, since the inlet control times are alwaysselected in such a way that only fresh gas is sucked in. A possiblyincomplete chemical reaction during exhaust-gas compression or expansionis implemented completely as a result of subsequent compression with ahigh effective compression ratio and subsequent conversion. Activationis used when cyclic combustion can no longer be maintained in the eventof a further increase in the fuel fraction in the fresh mixture or whenthere is exhaust-gas retention. This is recognized by erratic running ora load drop when the engine is in operation.

The logics in the control of ignition injection attempt to keep theignition injection quantity minimal with a view to a combustionconfiguration with optimum consumption. The control limits the maximuminjection quantity by means of the signal for air deficiency of theexhaust gas (air/fuel ratio ≦1) from the lambda probe in the exhaust-gastract.

Effective compression of the residual gas is dependent on theexhaust-gas fraction retained. It is, in most cases, at or below halfthe geometric compression. On account of the low fuel quantities foractivation and of the still relatively high exhaust-gas fraction, duringignition combustions the peak pressures and peak temperatures in theexhaust gas are also so low that nitrogen oxides cannot occur in anyappreciable quantity.

BRIEF DESCRIPTION OF THE DRAWINGS

Other features of the invention may be gathered from the further claimsand from the following description and also from the drawing whichillustrates diagrammatically exemplary embodiments of the invention andin which:

FIG. 1 shows a diagrammatic illustration of a reciprocating-pistoninternal combustion engine with a control unit for the gas-exchangemembers and the injection valves and with means for combustion andtorque analysis,

FIG. 2 shows a diagrammatic characteristic map of the distribution ofignition injection, main injection and after-injection against theengine load,

FIG. 3 shows a cylinder-pressure graph with the control times of thegas-exchange members and injection valves.

DETAILED DESCRIPTION OF THE DRAWINGS

The internal combustion engine illustrated diagrammatically in FIG. 1possesses a cylinder block 1 with four cylinders 2, in which pistons areguided sealingly and which are closed by means of a cylinder head. Thecylinder 2, piston and cylinder head enclose a combustion space 3, inwhich combustion takes place. A fuel injection valve 4, an inlet member5 and an outlet member 6 are located in the cylinder head for eachcombustion space 3. The gas-exchange members 5, 6 are opened and closedby an actuating device 7 and the fuel injection valve 4 is opened andclosed by an injection actuation system 8. A control unit 9 controls theopening and closing point of the gas-exchange members 5, 6 and of thefuel injection valve 4 continuously.

Combustion is monitored by means of an ionic current probe 10 in acombustion analyser 11 which is connected to the control unit 9. Saidanalyser is supplemented by a torque analyser 12 which detects therotational uniformity of the crankshaft on the circumference of aflywheel 13 with the aid of an engine-speed sensor 14 and transmits itto the control unit 9. In addition, a knock sensor may also be used toassess combustion. The ionic-current probe 10, engine-speed sensor 14and, if appropriate, knock sensor deliver real-time signals on theposition and profile of combustion for the control unit 9 which, takingthese values into account, brings about the control of the fuelinjection valve 4 and the gas-exchange members 5, 6.

FIG. 2 shows a diagrammatic characteristic map of the entire injectionquantities EM against the engine load ML, divided into ignitioninjection ZE, main injection HE and after-injection NE. Main injectionHE takes place in the case of any engine load ML, whereas ignitioninjection ZE is used only for a low engine load ML and after-injectionME only for a high engine load ML. Details of fuel injection areexplained in a subsequent operating description.

FIG. 3 illustrates a cylinder-pressure graph extending over a crankshaftangle of more than 720°, with the control times or control-time rangesof the gas-exchange members 5, 6 and fuel injection valves 4 and alsowith four strokes and six phases of the work cycle. The operation of themethod according to the invention will be explained with reference tothis cylinder-pressure graph:

In the four-stroke method, one stroke corresponds to one full pistonstroke. The first stroke 1T commences at gas-exchange dead centre LOT ata crankshaft angle of 0° and terminates at the first bottom dead centre1UT at a crankshaft angle of 180°. This is followed by the second stroke2T which terminates at the ignition dead centre ZOT. The third stroke 3Tthen commences, which terminates at the second bottom dead centre ZOT ata crankshaft angle of 540°. The subsequent fourth stroke 4T terminatesat the gas-exchange dead centre LOT at a crankshaft angle of 720°. Thenext stroke sequence then commences.

The six phases of the work cycle depend predominantly on the controltimes of the gas-exchange members 5, 6 and partially overlap from onestroke to the next.

The work cycle commences with an activation phase a, during whichexhaust gas retained in the region of the gas-exchange dead centre LOTis compressed and expanded between the outlet closing AS and inletopening EÖ. In the activation phase A, in the case of a low engine loadML, activation fuel is injected into the retained exhaust gas by meansof ignition injection ZE. In the case of a very low engine load ML, thistakes place during the compression of the activation phase a. The finelyatomized activation fuel evaporates in the hot retained exhaust gas andwith the residual oxygen of the retained exhaust gas forms an ignitablemixture. The residual oxygen was yielded by the lean mixture of theprevious work cycle. In the region of the gas-exchange dead centre LOT,the ignition of the activation fuel takes place, thus resulting in arise in temperature and pressure of the retained exhaust gas. Thepreconditions for reliable ignition of the main injection quantity inthe region of ignition dead centre ZOT are consequently afforded, evenin the case of a low engine load ML and when the engine is running cold.

Since there are only small quantities of activation fuel, which,moreover, burn the atmosphere containing a high proportion of exhaustgas, virtually no nitrogen oxide is formed at the same time. The rise intemperature of the retained exhaust gas makes it possible to reduce thequantity of the latter in favour of a greater quantity of fresh gas.

With an increase in engine load ML and engine temperature, ignitioninjection ZE is retarded. In the case of ignition injection at thegas-exchange dead centre LOT or thereafter, there is no ignition, but,instead a formation of radicals and the chemical activation of theactivation fuel, leading to the desired improvement in the ignition ofthe main fuel quantity. The partly burnt or activated activation fuel isburnt completely during main combustion, thus preventing the emission ofhydrocarbons.

Above a specific engine load ML and when the engine is running hot,ignition injection lapses due to the sufficient exhaust-gas fraction, asis clear from the illustration in FIG. 2.

Inlet opening EÖ is fixed for the point of shortfall of the ambientpressure P_(u) in the combustion space 3. This prevents a re-expansionof exhaust gas into the intake tract and consequently a cooling of theexhaust gas. The point of inlet opening EÖ and, consequently, thedeficiencies of the fresh charge capable of being sucked in and theengine load ML which is possible as a result depend on the point ofoutlet closing AS and on the quantity and injection point of ignitioninjection ZE. FIG. 3 therefore indicates, for outlet closing AS andinlet opening EÖ, a region where it is possible to adjust the engineload ML. The intake phase b and main injection HE commence with inletopening EÖ. The intake phase b extends until inlet closing ES. Thisoccurs when the ambient pressure P_(u) in the combustion space 3 isexceeded in the region of the first bottom dead centre 1UT. Maininjection HE extends beyond this point, depending on the engine load ML.Due to the advanced main injection HE, an intensive mixing of the mainfuel with the fresh charge and retained exhaust gas gives rise to ahomogeneous lean air/exhaust-gas/fuel mixture.

This is compressed, starting from the compression phase c commencing atinlet closing ES. In this case, the temperature of the homogeneous leanmixture rises to the ignition point of the latter. This is just beforeignition dead centre ZOT. Its position is controlled by the quantity andinjection point of the activation fuel and, if appropriate, by avariation in effective compression by means of variable inlet closing.

In the combustion phase d which follows ignition, self-maintaining oreven self-accelerating combustion takes place. Since the latter easilyleads to knocking combustion in the case of a relatively high engineload ML, simple or sequential after-injection NE into the compressionphase c is provided for this operating range. As a result, because ofthe relatively late fuel injection, the cold evaporation of the fuelgives rise to local mixture cooling with a subsequent ignition delaywhich results in knock-free combustion.

The combustion phase d is followed by the expansion phase e which isterminated by outlet opening AÖ in the region of the second bottom deadcentre 2UT. Outlet opening AÖ may be varied as a function of the engineload ML and of the engine speed in order to achieve maximum efficiency.The expansion phase e is followed by the expulsion phase f terminated bythe variable outlet closing AS which at the same time initiates the nextwork cycle.

A control loop which takes into account unintended fluctuations ofengine speed and engine load ML stabilizes the ignition point and theprofile of main combustion by means of the quantity and injection pointof the activation fuel. In this case, the control loop minimizes thequantity of activation fuel and limits the total fuel quantity to thestoichiometric value. A functionally reliable method for operating aninternal combustion engine with low fuel consumption and low pollutantemission is achieved in this way.

An alternative to the use of chemical activation is afforded by thecontrol of the ignition point by means of the position of the injectionpoint of main injection into the still retained exhaust-gas quantity, inthe case of sufficient volume, before the opening of the inlet members.

I claim:
 1. A method for operating a four-stroke reciprocating-pistoninternal combustion engine with a homogeneous lean basic mixture of air,fuel and retained exhaust gas and with compression ignition and directfuel injection into a combustion space, a volume of which changescyclically and which is capable of being filled with fresh gas throughat least one inlet member, and combustion exhaust gases being capable ofbeing expelled at least partially out of the combustion space through atleast one outlet member, comprising the steps of compressing andsubsequently expanding, in an activation phase of a work cycle, theretained exhaust gas in a region of gas-exchange dead center and, in aphase of the work cycle including the activation phase, injectingactivation fuel into the combustion space in order to stabilize maincombustion.
 2. The method according to claim 1, further comprising thestep of stabilizing the main combustion by a control loop, taking intoaccount unintentional fluctuations in a speed and load of the internalcombustion engine, by means of a quantity and injection point of theactivation fuel.
 3. The method according to claim 2, wherein the controlloop minimizes the quantity of activation fuel and limits a total fuelquantity to a stoichiometric value.
 4. The method according to claim 2,wherein the control loop influences an ignition point and a profile ofmain combustion by means of the injection point and quantity ofinjection of the activation fuel.
 5. The method according to claim 1,wherein, in a case of a low engine load and low engine speed and whenthe internal combustion engine is running cold, the activation fuel isinjected at a commencement of the compression of the retained exhaustgas, but, with an increase in engine load and engine speed and with arising engine temperature, the activation fuel is injected later and,beyond a specific engine load and engine speed and when the internalcombustion engine is running hot, is omitted.
 6. The method according toclaim 1, wherein the activation fuel is distributed homogeneously in theretained exhaust gas.
 7. The method according to claim 1, wherein anignition point is controlled by means of a position of an injectionpoint of main fuel injection into a still retained exhaust-gas quantity,in a case of sufficient volume, before opening of the at least one inletmember.
 8. The method according to claim 1, wherein a commencement ofthe compression of the retained exhaust gas is determined by a closingpoint of the at least one outlet member and a termination of theexpansion of the retained exhaust gas is determined by an opening pointof the at least one inlet member.
 9. The method according to claim 8,wherein the at least one inlet member is opened only after a pressure inthe combustion space has fallen short of an ambient pressure.
 10. Themethod according to claim 1, wherein an effective compression of theretained exhaust gas depends on a quantity of the retained exhaust gasand is below a maximum geometric compression.